An organic light-emitting diode (OLED), also referred to as an organic electroluminescent device, can be constructed by sandwiching two or more organic layers between first and second electrodes. The types of organic materials, thickness of vapor-deposited organic layers, and layer configurations useful in constructing an organic light-emitting device are described, for example, in commonly assigned U.S. Pat. Nos. 4,356,429; 4,539,507; 4,720,432; and 4,769,292, the disclosures of which are herein incorporated by reference.
Organic materials useful in making OLED displays, for example organic hole-transporting materials, organic light-emitting materials with an organic dopant, and organic electron-transporting materials, can have relatively complex molecular structures with relatively weak molecular bonding forces, so care must be taken to avoid decomposition of the organic material during synthesis, storage, transportation, and physical vapor deposition. The aforementioned organic materials are generally synthesized to a relatively high degree of purity, and are provided in the form of powders, flakes, or granules. It is known that providing and maintaining the organic materials in a state of relatively high purity is difficult.
Physical vapor deposition in a vacuum environment is the principal means of depositing thin organic material films as used in small molecule OLED devices. Such methods are well known, for example Barr in U.S. Pat. No. 2,447,789 and Tanabe et al. in EP 0 982 411. The organic materials used in the manufacture of OLED devices are often subject to degradation when maintained at or near the desired rate dependent vaporization temperature for extended periods of time. Exposure of sensitive organic materials to higher temperatures can cause changes in the structure of the molecules and associated changes in material properties.
To overcome the thermal sensitivity of these materials, only small quantities of organic materials have been loaded in sources and are heated as little as possible. In this manner, the material is consumed before it has reached the temperature exposure threshold to cause significant degradation. The limitations with this practice are that the available vaporization rate is very low due to the limitation on heater temperature, and the operation time of the source is very short due to the small quantity of material present in the source. In the prior art, it has been necessary to vent the deposition chamber, disassemble and clean the vapor source, refill the source, reestablish vacuum in the deposition chamber, and degas the newly-introduced organic material over several hours before resuming operation. The low deposition rate and the frequent and time consuming process associated with recharging a source has placed substantial limitations on the throughput of OLED manufacturing facilities.
A secondary consequence of heating the entire organic material charge to roughly the same temperature is that it is impractical to mix additional organic materials, such as dopants, with a host material unless the vaporization behavior and vapor pressure of the dopant is very close to that of the host material. This is generally not the case and as a result, prior art devices frequently require the use of separate sources to co-deposit host and dopant materials.
A consequence of using single component sources is that many sources are required in order to produce films containing a host and multiple dopants. These sources are arrayed adjacently with the outer sources angled toward the center to approximate a co-deposition condition. In practice, the number of such sources used to co-deposit different materials has been limited to three. This restriction has imposed a substantial limitation on the architecture of OLED devices. The requirement of many single component sources increases the necessary size and cost of the vacuum deposition chamber, increases the number of independent power supplies required to control the sources, and decreases the reliability of the system.
Additionally, the use of separate sources creates a gradient effect in the deposited film where the material in the source closest to an advancing substrate is over-represented in the initial film immediately adjacent the substrate while the material in the last source is over-represented in the final film surface. This gradient co-deposition is unavoidable in prior art sources where a single material is vaporized from each of multiple sources. The gradient in the deposited film is especially evident when the contribution of either of the end sources is more than a few percent of the central source, such as when a co-host is used. FIG. 1 shows a cross-sectional view of such a prior-art vaporization device 5, which includes three individual sources 6a, 6b, and 6c for vaporizing organic material. A vapor plume 9 is preferably homogeneous in the materials from the different sources, but in fact varies in composition from side to side, resulting in a non-homogeneous coating on a substrate 8.
A further limitation of prior art sources is that the geometry of the vapor manifold changes as the organic material charge is consumed, which requires that the heater temperature change to maintain a constant vaporization rate for a given organic material. It is observed that the overall plume shape of the vapor exiting the orifices can change with varying organic material thickness and distribution in the source.
For single component sources, the deposition rate determines the amount of vapor deposited on a substrate for a given length of time. In other words, the rate of vaporization of each individual deposition source is crucial because it determines the component ratio of the deposited organic layer on the substrate. Since the weight percentage of the dopant component in organic layers is lower than that of the host component, the deposition rate for the dopant component must be adjusted accordingly. If the rate of vaporization of individual sources is not precisely controlled, the component ratio of the deposited layer can vary significantly from the optimum for a given OLED display.
Powders, flakes, or granules typically have high surface to volume ratios, and a correspondingly high propensity to entrap air and/or moisture between particles. Consequently, a charge of organic powders, flakes, or granules loaded into a physical vapor deposition source within a chamber often must be thoroughly outgassed by preheating the source at reduced pressure once in the chamber. If outgassing is omitted or is incomplete, particulate can be ejected from the evaporation source during the physical vapor deposition process. An OLED, having multiple organic layers, can become inoperative if such layers include particles or particulates. Further, the aforementioned aspects of organic powders, flakes, or granules can lead to non-uniform heating of such organic materials in physical vapor deposition sources, causing spatially non-uniform vaporization of organic material. This can result in non-uniform layers on an OLED device. Moreover, undesired contaminants can be vapor-deposited on a structure. There is a need, therefore, for a method of purifying various organic materials employed in OLED devices, providing a plurality of organic materials in a purified state to a thermal physical vapor deposition system, and co-depositing the organic materials on a substrate.